A thin film capacitor employing a ferroelectric film has been conventionally utilized as a storage cell taking advantage of its reinanent polarization. One example of the storage cell is shown in FIG. 9 with its equivalent circuit and in FIG. 10 with its cross section.
In FIG. 10, numerals 12 to 15 respectively
represent a ferroelectric film, a lower electrode, a diffusion layer and a gate electrode. Numerals 16 to 18 respectively represent a first insulating film, a second insulating film and a film wiring made of aluminum. Numerals 19 and 20 respectively represent a field oxide film and an upper electrode. In a storage cell of this constitution, one transistor 22 and one capacitor 21 comprising the ferroelectric film 12, the lower electrode 13 and the upper electrode 20 are connected with each other. When a sufficient positive or negative voltage higher than coercive voltage is applied to the capacitor 21, the positive or the negative reinanent polarization is realized due to hysterisis of the ferroelectrics. By applying a pulse voltage of constant polarity to the polarized capacitor 21, information which has caused the reinanent polarization can be read out.
The switching time for reversing polarized direction of the ferroelectric film capacitor is generally given by the following formula. ##EQU1## where, .alpha. is an activation electrical field t.sub.o, k and n are respectively a certain constant, and E is an electrical field. As is obvious from the formula, in order to vary the switching time the applied electrical field must be varied. Thus, there is a problem that with respect to one sample the switching time can not be varied unless the electrical field is varied.
On the other hand, for realizing rapid operation of a storage cell there is some requirements for more rapid switching time of a ferroelectrics capacitor.